The pattern in geographical distribution of some species is discontinuous. For example, grey seal is a species whose geographical distribution is disjunct due to the large discontinuity between populations in the eastern and western Atlantic and is related with environmental factors.
Another example is fish spawning. Salmon migrates from seas to rivers to lay eggs and eel at the same time migrates from rivers to the Sargasso Sea.
People often introduce species to new areas to achieve a particular objective or by chance. Such situation happened with Leptinotarsa decemlineata. It is obvious that usually such introduction has a destructive effect on the ecosystem and local species suffer from it. In general, these actions also contribute to discontinuity in the distribution.